Research Summary Microsporidia are a phylum of obligate intracellular pathogens within the fungal kingdom that infect hosts ranging from invertebrates to humans. In humans, microsporidia cause potentially fatal wasting diarrhea. Latent infection by microsporidia is a common phenomenon, with up to 11% of the immune-competent population asymptomatically infected and transiently shedding spores. The full impact of these pathogens on human health is unknown. There is also a critical lack of effective drugs to treat microsporidiosis. Only one drug is approved for treating humans, albendazole, which is ineffective against the leading cause of microsporidiosis, Enterocytozoon bieneusi. Determining the molecular mechanisms of microsporidian pathogenesis is critical for understanding how these pathogens cause disease and for developing new strategies for therapeutic intervention. Adaptation of microsporidia to this intracellular parasitic lifestyle has resulted in extreme gene loss and genome compaction, including the loss of conserved signaling components responsible for controlling proliferation and sporulation in other fungi. In addition, this obligate intracellular lifestyle of microsporidia has severely hampered the ability to propagate and study these organisms and is the primary reason for such a poor understanding of these pathogens. However, the discovery of Nematocida parisii, a natural pathogen of the model nematode Caenorhabditis elegans, provides for the first time a tractable system for understanding the molecular mechanisms of microsporidian pathogenesis in a whole animal model. I hypothesize that microsporidia co-opts host cell signaling pathways to gauge host cell conditions and control proliferation and differentiation into spores. I will take advantage of the N. parisii/C. elegans infection system to identify the host signals that control microsporidian spore production. I will use an RNAi screen to identify host genes that contribute to the production of N. parisii spores. Then I will analyze candidate developmental regulators of N. parisii sporulation to determine their stage-specific expression. Lastly, I will determine the host factor dependence of these N. parisii transcription factor networks. Ultimately, these studies will shed light on developmentally regulated genes in microsporidia; genes important for the formation of specialized infectious spores. Many of these developmentally regulated genes will likely contribute to host cell egress and host cell invasion machinery, which are attractive therapeutic targets to prevent and limit disease caused by microsporidia.